Method and apparatus for monitoring and optimizing blood circulation generated by a pump

ABSTRACT

In a method for the automatic control of blood pumps, an optimization of the blood flow is achieved by periodic speed interventions and flow changes thereby occurring, using a formed differential variable and a control algorithm. In addition, the location of possible flow resistances on the venous or arterial side can be ascertained.

RELATED APPLICATIONS

This application is a National Stage Entry (under 35 U.S.C. 371) ofPCT/EP2008/000927 filed Feb. 7, 2008 which claims the benefit ofpriority to German Application No. 10 2007 007 198.3 filed Feb. 9, 2007,the disclosures of which are hereby incorporated by reference in theirentirety.

The present invention relates to a method for monitoring and optimizingautomatically the blood flow of a pump. Conclusions can be drawn fromthe control parameters as to the patient's condition and any problems inthe blood circulation generated by the pump. This ensures that asufficient blood flow takes place at all times.

The invention also relates to a medical apparatus which comprises a pumpcontrol system according to this method.

Blood pumps and associated control systems have long been known in theprior art. The task of blood pumps in combination with associatedcontrol systems is to ensure blood circulation intra- orextra-corporeally and thus to support or replace the pumping function ofthe patient's heart. Such blood pumps are used for a limited time toreplace the heart function during operations on the heart or to supportthe heart function as a possible means for recovery of the weak heart inan extracorporeal circuit. Apart from the blood pump, use may be made,depending on the requirements, of all products commonly used for theextracorporeal circuit, such as for example cannulas, catheters, tubesand connectors, reservoirs, oxygenators, heat exchangers, bloodconcentrators and dialysers, bubble traps and filters.

Moreover, applications of heart support for an unlimited time and alsoartificial hearts are known, implantation of the pump taking place inthese cases.

In all these applications of blood pumps, it is of utmost importance togenerate reliably a sufficient blood flow by means of the pump.Especially in the case of the longer-term use of the blood pump, it isnot possible manually to monitor and, if need be, correct the blood flowand associated parameters such as venous or arterial blood pressure orgas flow and gas mixture with the simultaneous use of oxygenators.

A reliable automatic control of the blood pump is therefore requiredespecially for the use of ECMO (extracorporeal membrane oxygenation) orELS (extracorporeal life support), since these applications take placeover a fairly lengthy period of days up to weeks and also outside theoperating theatre and without staff-intensive monitoring. But even inthe case of applications in heart surgery, an automatic pump controloffers greater convenience in handling and greater safety in the case offlow disruptions. Moreover, such a control is also conceivable in thecase of implantable support systems (ventricular assist devices, VAD's)or in the case of an implantable artificial heart.

In the following, the factors are dealt with that adversely affect adesired blood flow, so that the pump settings and possibly additionalparameters have to be changed.

For the venous blood return flow, it is known from practice and fromBaloa et al., Vollkron et al. that, in the case of insufficient venousreturn flow of the blood, the large venous vessels and/or the atrium ofthe heart (depending on which one is being drained, the left or theright atrium) may collapse and then no blood flow at all or only aninsufficient blood flow is possible. This venous return flow depends,amongst other things, on the filling status and the position andconstruction of the venous catheter. In the case of an excessively smallfilling status or in the event of contact of the catheter opening withthe vessel wall, the vessel collapses and obstructs the venous andtherefore the whole blood flow. This phenomenon is known in connectionwith centrifugal pumps, but can occur independently of the pumpingprinciple. Furthermore, impairments of the venous return flow due tothromboses or suchlike or kinking of the tube in the venous line areconceivable. If such a state of insufficient venous return flow isreached, not only insufficient blood flow but also damage to the bloodcan occur due to cavitation and formation of gas bubbles. Thislife-threatening and patient-endangering state therefore has to bereliably avoided.

The monitoring of the pressure on the venous line is not sufficient toavoid collapse, because negative pressures can occur both in the case ofcollapse as well as in the operationally safe state. Moreover, thepressure on the venous side of the blood pump is to a large extentdependent on its position in relation to the patient (hydrostaticpressure) and can therefore be influenced thereby.

Furthermore, a collapse that has already occurred can at best bedetected in this way and the tendency towards collapse can only bedetected with difficulty.

In the arterial line, it is exclusively obstructions or closures due forexample to thromboses or embolisms or kinking of the tube that areconceivable as an impairment of the blood flow. The problem of thecollapse of the vessel wall does not exist on account of the arterialpressure. The level of the arterial pressure alone is not suitable as ameasure for the detection of flow disruptions, because a high arterialflow resistance can be caused by the stated flow obstructions in thearterial line or by a high vascular resistance.

Approaches are disclosed in the prior art which characterize thisproblem mathematically. Baloa et al. disclose for example the method offorming the differential variable DRI (diminishing return index)DRI=dQ/dω by gradually increasing the pump speed, measuring therespective flow and differentiating by the pump speed. In the ideal caseof unobstructed flow, this variable is a constant. With an incipientcollapse, DRI becomes smaller, reaching the value 0 when collapseoccurs. It is not however disclosed how a possible collapse is to beavoided or how a collapse that already exists is to be removed.

The method employed in many VAD's on the market of simply switching offthe pump for several seconds in such cases appears to involve a highdegree of risk due to the sudden total removal of the pump function andtherefore the circulation support. In this case, the weakened patient'sheart suddenly has to take over the whole burden of the bloodcirculation without a process of adaptation, and this can lead to itbeing overloaded and therefore to additional damage. In addition, thecollapse can reoccur at any time with the described consequences whenthe original speed is restored. If, for the sake of safety, a lower pumpspeed is instead preset, the smaller flow thereby generated can possiblybe maintained in a stable manner. Then, however, there is a reduced flowcompared to the basic setting, said flow not being able to produce thefull intended circulation support. In addition, the applicationdescribed by Baloa et al. is used in a VAD, wherein it is not a vessel,but rather the atrium that collapses.

Vollkron et al. ascertain the venous return flow with the aid of thepulsatility in the flow signal, which is only present with a beatingheart. In the case of an excessively high pump speed, the latter isreduced slowly down to a minimum, after which the speed is slowlyincreased again up to a maximum safe and desired flow. This slowdecrease and increase, however, requires a certain period of time inwhich the pump seeks the optimum flow, but during which the optimumsupport is not guaranteed.

Against this background, the problem underlying the present invention isto make available a method for automatically regulating and optimizing,within the range of the desired preset values, the blood flow from bloodpumps whilst avoiding life-threatening states, wherein the method isintended to be universally usable, i.e. one that functions reliably evenin the event of cardiac arrest, and in the case of corrected impairmentsof the blood flow it should be possible to detect their location in thecirculation, venous or arterial, in order to permit rapid measures to betaken in a targeted manner.

This problem is completely solved by the method according to theinvention and a device for performing the method.

This new method can automatically perform periodic interventions intothe pump speed, wherein differential variables are ascertained bydetermining the effect on the flow by recording the measured values ofpump parameters such as the speed and the blood flow per unit of timeand their relationship with one another, said differential variablescharacterising the flow states and being suitable for detecting andremoving return flow disruptions and for analyzing the state of theheart.

The blood flow required depending on the patient's needs is establishedin a known manner by the doctor in charge or perfusionist and thenumerical magnitude “¾ of the blood flow required in the circulation”required for the method according to the invention is thus also known.For example, the required blood flow can be calculated using the du Boisformula for a patient-related body area, cardiac indices in the range of2.5-4 l min⁻¹ m⁻² being normal.

In contrast to Baloa et al., the pump speed is not only increased, butalso decreased and the changed blood flow is measured. In this way, if atendency towards collapse is detected, it is possible by reducing thepump speed to achieve a sufficient venous return flow again withregeneration of the incipient collapse. Thereafter, the speed and theflow can be optimized gradually by increasing the speed and measuringthe changed flow (blood flow), stable values above the initial value forincipient collapse being able to be achieved.

The method according to the invention, wherein an incipient collapse isnot only detected, but also rapidly eliminated and the flow thereafteroptimized, differs markedly from the prior art (Baloa et al., Vollkronet al.), where differential values are also ascertained, but no solutionis disclosed for the regeneration of an incipient or existing collapse.The method employed in many VAD's on the market of simply switching offthe pump for several seconds in such cases appears to involve a highdegree of risk due to the sudden total removal of the pump function andtherefore the circulation support. In this case, the weakened patient'sheart suddenly has to take over the whole burden of the bloodcirculation without a process of adaptation, and this can lead to itbeing overloaded and therefore to additional damage.

On the other hand, the removal of the collapse disclosed by Vollkron etal. requires a certain amount of time until the pump speed has beenreduced, and then also increased again. In particular, the principleaccording to the invention is distinguished here by a rapid slowing-downof the pump with a subsequent increase in the pump speed, this functionalso being present with a non-pulsatile blood flow, e.g. in the case ofcardiac arrest.

The method according to the invention and a device with which the methodaccording to the invention can be performed is described with the aid ofthe following figures in the drawing.

In the figures:

FIG. 1 shows a diagram of a minimized extracorporeal bypass with a pump,oxygenator and hose lines;

FIG. 2 shows a flow diagram with the illustration of the main steps ofthe algorithm;

FIG. 3 shows a diagram of a control circuit according to the invention,wherein the setpoint value is a range of permitted values of thedifferential flow speed ratio (DFSR), which allows the control system tocontrol the pump speed (PS) and accordingly generates a flow, a maximumflow also being able to influence the DFSR control, i.e. to prevent anover-perfusion;

FIG. 4 shows a graphic relationship between changed pumping capacity andascertained DFSR values; and

FIG. 5 shows a control sequence represented in an illustration, such ascan preferably take place according to the invention.

In FIG. 1, a patient is indicated by 10, who is connected to anextracorporeal blood circuit, comprising a pump 12, an oxygenator 14, avenous hose line 16 and an arterial hose line 18.

During the initial phase of a minimized closed extracorporeal circuitsystem (FIG. 1), the pump speed (PS) is gradually increased by αrev/min. The value “differential flow speed ratio” (DFSR) is constantlycalculated via Δ_(flow)/Δ_(speed) (=Δflow/ΔPS [l/min.][revolutions/min]) until ¾ of the required flow is reached. The meanvalue of the calculated differential represents an approximate value ofthe flow/speed ratio for each individual patient with circulationsupport. This value is halved and used as threshold value T (defined).During the bypass, the pump speed (pump speed, PS) is reduced by βrev/min for ε seconds. Following the pump speed reduction (dPS), theflow change (dF) is calculated and the differential variable“differential flow speed ratio” (DFSR), i.e. Δflow/ΔPS, is ascertainedtherefrom.

DFSR≦1T signifies an excessive drainage, i.e. an excessively smallreturn flow, and requires a sudden regulation of pump speed PS down to50% of the initial pump speed. After γ seconds, PS gradually increasesby δ rev/min up to 95% of initial pump speed PS. DFSR>2T signifies anexcessively small drainage, which requires an increase in pump speed PSby ε rev/min. The calculation of the new DFSR shows the success of thereturn flow optimization.

The value of 1.5T is taken as the upper threshold value. DFSR>1.5Tindicates that a higher pump speed and therefore also a higher flow ispossible. When the option is activated, DFSR>1.5T can cause the pump toincrease the speed by ζ rev/min. The calculation of the new DFSR showsthe success of the return flow optimization. If it is desired to avoid apossible over-perfusion, a maximum flow can be inputted, which thenreceives priority over the speed increase.

A diagram of the control unit described above is illustrated in FIGS. 2and 3. FIG. 4 shows results of a practical application.

FIG. 2 shows, in a flow diagram, the inventive procedure (algorithm) forachieving an optimum drainage. After the start of the extracorporealblood circulation, the necessary patient-related limiting values areascertained, as described by way of example in respect of FIG. 1. Thevenous blood return flow is then quantified via a change in the pumpingcapacity and the pumping capacity enabling optimum drainage isascertained therefrom and set.

FIG. 3 represents a control circuit according to the invention, whichconstantly ascertains, via desired DFSR values, new DFSR actual valuesfrom the instantaneous flow of the pump speed, matches the DFSR actualvalues with the desired DFSR values and from this constantly resets thepump speed in order to prevent both an excessive drainage as well as adrainage that is too small.

FIG. 4 illustrates how a mean value is formed (inst. DFSR) afterincreasing the pump speed and simultaneously measuring the DFSR. Half ofthis value is set as a limiting value. After the DFSR has beenascertained, the venous return flow is reduced (represented by thearrow). A subsequent measurement to ascertain the DFSR ascertains areduction of the same, after which the pump speed is effectively sloweddown to 95% by means of a 2-step reduction. A subsequent measurement ofthe DFSR clearly shows an increase in this value and an optimization ofthe drainage, i.e. adaptation of the pump speed to the available venousflow.

Furthermore, process steps a and b from process claim 1 as originallyfiled are entered in FIG. 4, process step d being indicated by a furtherarrow.

Return flow obstructions increase the flow resistance on the inlet sideof the pump, and this reduces the pump preload and adversely affects thepump flow. In the case of clotting of an oxygenator, the resistance issimilarly increased, but this additional resistance then occurs afterthe pump. Both cases influence the DFSR, but in order to distinguishbetween clotting of the oxygenator and obstruction of the venous returnflow to the pump, the additional measurement of the drop in pressureacross the oxygenator (pressure at inlet−pressure at outlet) can providethe required information. If the DFSR falls, the level of thedifferential pressure on the oxygenator indicates the cause of theincreased resistance, i.e. whether it is before or after the pump. Ifthe differential pressure is small, an obstruction of the venous returnflow is present, if the differential pressure is high, clotting of theoxygenator has occurred.

Furthermore, the measurement of the DFSR provides information concerningthe pumping capacity of the heart, in particular the right chamber ofthe heart. In the case of a recovering heart or a mean pumping speedthereby reduced, the DFSR should not change. The recovering heartreacts, according to the Frank Starling mechanism, to the increasedpreload and tries to achieve the required output capacity. The venousand arterial pressure, and therefore the DFSR, thus remain constant. Inthe case of an insufficient and only partially recovered heart, areduction in the mean pump speed leads to an increase in the DFSR. Theinsufficient heart is not capable of overcoming the increased preloadwhen the pump support is reduced. In this case, therefore, the venouspressure increases and the arterial pressure diminishes. The body inturn reacts to the falling arterial pressure with vasoconstriction, inorder to compensate for a further drop in the arterial pressure. Thepreload of the pump increases as a result, whilst the after-load remainsapproximately constant: the DFSR increases.

The reaction to speed-reducing control steps can be used, in combinationwith a flow pattern analysis, to ascertain the heart status. The beatingheart influences the flow pattern of a supporting centrifugal pump, insuch a way that the aortic pressure increases during systole, and theflow via the pump thereby diminishes. If the mean pump speed is reducedand speed-reducing control steps then follow, the following pulses ofthe flow signal provide valuable information concerning the heart statusand its pumping reserve. After reduction of the support, the gradientand the amplitude of the flow pulses provide direct informationconcerning the contraction capacity and therefore the regeneration ofthe heart.

The above methods can be used to determine the heart status and thereserves of the pumping capacity, and can be used in an appropriate wayto wean the patient off the heart support. They can be used in a veryhelpful way as a tool for heart monitoring, while the latter receivesmore preload via the periodic speed reductions and is thereby trained.

Another possibility for ascertaining the heart status and heartregeneration is to determine the time that the heart requires tocompensate for abrupt pump support and to deliver the total flowrequired by the body, consisting of pump flow and cardiac output.

The Frank Starling mechanism is used directly in both methods. However,this application requires, in contrast to the non-invasive determinationof the DFSR, an invasive measurement of the arterial and venous bloodpressure.

In this way, it is also possible to automate the monitoring and theoptimization of the blood flow, this being necessary for heart supportand regeneration in the case of a long-term application. Moreover,indications can be obtained concerning the recovery of the heart throughthe reaction of the heart to a changed blood flow via the pump. Thehigher the pulsation in the arterial blood pressure with a reduction inthe pumping capacity, the greater the capacity and therefore therecovery of the heart. The required support can therefore be givenautomatically during the time of the heart support, the pumpautomatically being able to assume a greater share of the pumpingcapacity in the event of a reduction of the pumping capacity of theheart. A risk-free regeneration of the heart and thereafter automaticcontinuous weaning off from the pump is thus possible.

FIG. 5 shows, by way of example, how a control according to theinvention can take place. After the required blood flow has beenascertained, this being a procedure familiar to the person skilled inthe art addressed here, the control procedure commences and ascertainsthe DFSR. A fraction thereof, in the example here half thereof, isdefined in the manner described as threshold value T.

In FIG. 5, the straight line for DFSR=1T is represented with 22. Thecourse of DFSR=1.5T is shown with reference number 24 and DFSR=2T with26. Operational area 28 of the method according to the invention isindicated between straight lines 22 and 24. Below straight line 22(DFSR=1T), the undesired operational area is marked as grey area 30(DFSR<1T), in which partial or complete collapse can occur. This area iscritical for the patient's safety and it is therefore absolutelyessential for it to be reliably avoided.

When DFSR≧1.5T, the pump speed is increased, when DFSR≦1T, it is reduced(and thereafter increased again). A stable operation in the operationalarea between 1T and 1.5T is thus achieved.

The starting points of straight lines DFSR=1T, DFSR=1.5T and DFSR=2T donot begin at the origin in the above exemplary diagram. It goes withoutsaying that, depending on the existing pressure conditions during usewith a centrifugal pump mentioned by way of example, a positive flow >0generated by this pump cannot be brought about until a given speed PSand the determination of DFSR cannot therefore begin until this speedand ends at ¾ of the desired flow. The values for the speed (PS)occurring in the pump circuit and flow >0 up to ¾ of the desired flowgenerated by the pump (=positive) thus have to be evaluated in order toascertain the DFSR and thus T.

For a general understanding, reference is once again made to the factthat the DFSR is formed via the variables Δflow/Δspeed for a sequence ofdifferent speeds. This value, however, is therefore to be understood asa differential variable. The DFSR therefore represents the 1^(st)derivative (gradient of the function Δflow/Δspeed at each point) of therespective function.

For unchanging return flow conditions, i.e. with an almost ideal,unobstructed blood flow, a straight line is to be expected for thisfunction and a constant DFSR variable as a derivative thereof. Thisbehaviour is used to ascertain the inst. DFSR and thus threshold valueT.

A change in this gradient, equivalent to a changed value for the DFSR inthe considered range of the pump speed, indicates a change in the flowconditions.

In the method according to the invention, periodic speed interventionsare carried out from a given pump speed and the differential variableDFSR for the region of this intervention is formed from Δflow/Δspeed forthe upper and lower considered speeds of the intervention.

By means of a comparison with threshold value I (fraction of inst. DFSR,in the special case ½ inst. DFSR), the pump speed is regulated in such away that flow conditions are kept within a preset range for the DFSR.This preset range from T to 1.5T characterizes, in this example ofembodiment, pump speeds adapted to the available return flow volume,over-drainage being avoided on the one hand and the predeterminedrequired blood flow under valuable drainage conditions being achieved tothe best possible effect on the other hand.

It does not therefore concern a control procedure for the blood flow ingeneral, but rather the adaptation of the pump speed to the flowconditions with a predetermined target value for the maximum blood flow.

In a development of this method, the gas supply of an oxygenator presentin the blood circuit can be integrated into the control circuit. Sincethe blood flow is adjusted variably by regulating the pump, the gas flowand the oxygen content of the gas flow must also be automaticallyadapted to the given blood flow and the patient's needs. For thispurpose, it is proposed to use the existing blood flow and the partialpressures of blood gases pO₂ and pCO₂ measured online to regulate thegas supply. If both partial pressures are normal, the gas mixture andthe gas flow can be maintained. If pO₂ is too small, the oxygen contentof the supplied gas mixture must be automatically increased and viceversa. An excessively high value of pCO₂, on the other hand, is adjustedby an increase in the gas flow, whilst an excessively low value of pCO₂is adjusted by reducing the gas flow. The algorithm can, analogous tothat proposed for the blood flow, be constituted with threshold orlimiting values.

In this case, the automatic detection of the differential pressure onthe oxygenator and a warning signal when a threshold value is reachedfor the differential pressure or gas partial pressures is of particularimportance. The risk of an incipient under-supply to the patient forheart and lung function can thus be detected and removed by suitablemeasures, such as for example replacement of a clotted oxygenator.

Although only measured values of centrifugal pumps have been disclosedin FIG. 4, it is evident to the person skilled in the art that theproblem of the venous return flow and the risk of the collapse ofvessels or ventricles can in principle occur with the use of blood pumpsemploying any functional mode and can be solved according to theinvention.

It goes without saying that, with the above non-invasive methods, notonly the indicated flow of the pump can be used as a parameter, but alsothe variables of the pump proportional to this flow, such as for examplethe power or current consumption of a centrifugal pump or the current orthe power consumption of active magnetic bearings of axial pumps, can beused as control parameters, without departing from the scope of theinvention.

In a method for the automatic control of blood pumps, an optimization ofthe blood flow is achieved by periodic speed interventions and flowchanges thereby occurring, using a formed differential variable and acontrol algorithm. In addition, the location of possible flowresistances on the venous or arterial side can be ascertained

A method for controlling blood pumps, characterized in that thedifferential variable “differential flow speed ratio” (DFSR) is formedaccording to Δ_(delivery capacity)/Δ_(speed/pump speed) by automaticperiodic speed changes or pump speed changes and the change in thedelivery capacity of the pump measured for this, the level and course ofsaid differential flow speed ratio being used in an algorithm whichbrings about the optimization of the blood flow.

A method characterized in that the automatic periodic speed or pumpspeed changes can be both increases and decreases.

A method characterized in that the change in the delivery capacity isdetermined by a change in pump parameters.

A method characterized in that the determination of the speed or pumpspeed changes and the change in the delivery capacity of the pumpmeasured for this take place non-invasively.

A method characterized in that the determination of at least oneparameter takes place invasively.

A medical apparatus which contains a pump for delivering blood with acontrol system.

The invention claimed is:
 1. A method for operating blood pumps of anintra- or extra-corporeal blood pump circuit with the following processsteps: a) determining a threshold value T from differential flow speedratio variables of blood flowing through the blood pump circuit (DFSRvariables), wherein individual DFSR variables are calculated from aquotient of a differential blood delivery capacity with respect to acorresponding differential blood pump speed, wherein the pump speed isgradually increased up to a value less than a predetermined blood flowof the blood pump circuit when the DFSR variables are obtained; b)subsequently reducing the blood pump speed and obtaining associated DFSRvariables of blood flowing through the blood pump circuit while reducingthe blood pump speed (determined DFSR variables); c) comparing thedetermined DFSR variables from process step b) with the threshold valueT from process step a) in order to assess drainage flow of the bloodpump circuit; d) changing the blood pump speed depending on whether thedetermined DFSR variables according to process step b) is greater than,equal to or less than the threshold value T to regulate blood flowthrough the blood pump circuit.
 2. The method according to claim 1,characterized in that the threshold value T is half a mean value of theDFSR variables.
 3. The method according to claim 1, characterized inthat, when the determined DFSR variable≦1T, the blood pump speed isreduced and thereafter is increased over a defined period of time. 4.The method according to claim 1, characterized in that, when thedetermined DFSR variable≧2T, blood pump speed is increased.
 5. A devicefor performing a method according to claim 1, characterized in that theblood pump circuit comprises a first means for detecting, calculatingand comparing the determined DFSR variables and a second means forsetting the blood pump speed as a function of a result from a comparisonof the determined DFSR variables with the threshold values T.
 6. Amethod for operating a blood pump of a blood circulation system toregulate blood flow comprising the steps of: determining a minimumthreshold value based on a differential flow speed ratio of bloodflowing through the blood circulation system over a first period of time(DFSR); comparing: (a) a determined differential flow speed ratio ofblood flowing through the blood circulation system obtained over asecond period of time when pump speed is decreased (determined DFSR) to(b) the minimum threshold value in order to assess drainage flow of theblood circulation system; and adjusting the pump speed to regulate bloodflow through the blood circulation system by first decreasing the pumpspeed to a non-zero magnitude and subsequently increasing the pumpspeed, wherein the step of adjusting occurs in the event the determinedDFSR is less than the minimum threshold value.
 7. The method of claim 6,wherein the step of adjusting the pump speed comprises decreasing thepump speed at a first rate of change to a non-zero magnitude andsubsequently increasing the pump speed at a second rate of change,wherein the second rate of change is smaller than the first rate ofchange.
 8. The method of claim 6, wherein the minimum threshold value isassociated with a condition of impeded drainage; and wherein the step ofadjusting the pump speed comprises decreasing the pump speed asufficient amount to restore stable drainage of blood through a vesseland subsequently increasing the pump speed a sufficient amount tostabilize blood flow through the vessel.
 9. The method of claim 8,wherein the step of adjusting the pump speed comprises minimizing orpreventing backflow during the step of first decreasing pump speed to anon-zero magnitude.
 10. The method of claim 6, wherein the step ofincreasing the pump speed comprises increasing pump speed to a magnitudeless than a pump speed corresponding to a detection that the determinedDFSR is less than the minimum threshold value.
 11. The method of claim6, wherein the step of adjusting pump speed comprises decreasing pumpspeed and subsequently increasing pump speed such that a correspondingdifferential flow speed ratio is greater than the determined DFSR. 12.The method of claim 6, wherein the step of regulating blood flowcomprises decreasing pump speed down to 50% of a pump speed correlatedto a detection that the determined DFSR is less than the minimumthreshold value.
 13. The method of claim 6, wherein the step ofregulating blood flow comprises increasing pump speed up to 95% of apump speed correlated to the detection that the determined DFSR is lessthan the minimum threshold value.
 14. The method of claim 6, furthercomprising the steps of: determining a maximum threshold value based onthe DFSR; comparing the determined DFSR to the maximum threshold value;and increasing pump speed in the event the determined DFSR is greaterthan the maximum threshold value.
 15. The method of claim 6, wherein theminimum threshold value is determined based on a mean DFSR obtained overthe first period of time, wherein pump speed is increased during thefirst period of time.
 16. The method of claim 15, wherein the minimumthreshold value is the mean DFSR.
 17. The method of claim 14, whereinthe maximum threshold value is determined based on a mean DFSR obtainedover the first period of time, wherein the pump speed is increasedduring the first period of time and wherein the maximum threshold valueis ≧2 times the mean DFSR.
 18. The method of claim 6, further comprisingthe step of assessing cardiac pump capacity or cardiac recovery based onthe step of comparing.